Recently, higher speed and capacity of an optical communication system has been progressing, and thus using a communication speed of 40 gigabit/second or higher per one wavelength has been considered practical. According to this, band-widening of the optical modulator, which is a basic component, has been demanded. A traveling-wave type optical modulator is an optical modulator that modulates optical waves by an interaction between optical waves that travel through the optical waveguide and microwaves that travel through an electrode provided along the optical waveguide, the interaction being attributed to an electro-optical effect, and the band-broadening is realized by speed matching between the optical waves and the microwaves. As a method of realizing the speed matching, a configuration in which an electrode is formed on a low-permittivity buffer layer provided on an optical waveguide substrate has been used in the related art. However, in this configuration, since an electric field that is applied to the optical waveguide decreases due to the presence of the buffer layer, there is a disadvantage that voltage reduction of a drive voltage may not be realized.
To correct the disadvantage, there is suggested a travelling-wave type optical modulator in which the optical waveguide substrate is made to be thin as shown in FIG. 5 (for example, refer to PTL 1). In FIG. 5, an optical waveguide substrate 101 in which an optical waveguide 104 is formed is fixed and held to a holding substrate 102 by an adhesive layer 103. The thickness of the optical waveguide substrate 101 is approximately 10 μm or less, and is smaller than that of a common substrate (for example, thickness of 0.5 mm). As the adhesive layer 103, a layer having permittivity lower than that of the optical waveguide substrate 101 is used, and the thickness of the adhesive layer 103 is made to be sufficiently large (for example, 10 μm to 200 μm) in order for leakage of an electric field applied from an electrode 105 to the adhesive layer 103 to increase. In this configuration, the electric field from the electrode 105 is leaked to the inside of the adhesive layer 103 having a low dielectric constant, and thus an equivalent refractive index (a value thereof is larger than an equivalent refractive index with respect to optical wave) with respect to microwaves becomes smaller compared to a case in which the thickness of the optical waveguide substrate 101 is large. In this manner, since a difference in a value of the equivalent refractive index decreases, it becomes close to a state in which the speeds of the optical waves and the microwave are matched to each other, whereby band-broadening is realized. Along with this, in this configuration, the speed matching becomes possible without providing a buffer layer on the optical waveguide substrate 101, and thus the intensity of the electric field applied to the optical waveguide 104 does not decrease, and the voltage reduction of the drive voltage may be realized at the same time.
However, in the configuration shown in FIG. 5, since the thickness of the adhesive layer 103 is large, the following problems occur. First, if the thickness of the adhesive layer is large, the bonding strength thereof is apt to decrease. Second, since the temperature is increased by ultraviolet irradiation or heating and lowered after being cured, stress occurs during the curing of the adhesive, and if the adhesive layer is thick, occurrence of the stress is apt to increase. Third, manufacturing cost is increased, since a process for forming a thick adhesive layer involves difficult manufacturing steps, such as correcting of parallelism of a substrate and prevention of dripping.
As a technology of coping with these problems, there is suggested a structure of using a resin substrate as disclosed in PTL 2 and PTL 3. According to this structure, since a thick adhesive layer is not used, there are merits in a process or characteristic aspect such as ease of carrying out the correction of parallelism during bonding and a small effect due to shrinkage of an adhesive during curing.